Is there any clear definition of game-ready models?
Game-ready is a very vague definition. But across the industry it usually means being very aware of the performance impact of polygons and textures and use them as efficiently as possible:

Geometry:


Mesh is all triangles or quads(which can be easily converted to triangles), depending on engine. No ngons for sure.  Avoid high valence vertex.
Smart use of triangle budget to ensure optimal performance.
A lot can be done with shaders and textures these days, so the geometry is likely going to be quite simple.


Textures:


Maximize use of UV space, wasted texture space is wasted memory.
Textures in power of 2 (1024x1024, 4096x4096)
Use texture size that's appropriate for the size of the object on the screen.
Usually a single object has a single material and single textureset(1 albedo, 1 normal, 1 roughtness map for a typical PBR workflow). This ensures that the object can be drawn with one drawcall for optimal performance.


Workflow:


Note most of these requirements only apply to the end result, as you are creating your models, you can use multiple textures, multiple modifiers, even ngons temporarily, until you are ready to export the final model.
Most game models benefits from having a detailed normal map, which uses a low-poly mesh combined with a normal map to represent additional surface bumps. So often you'd have to model the geometry twice, once as a high poly model, and once more as a low poly model, in order to create a baked normal map.